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Myelin takes time to develop:

Teens can't control impulses and make rapid, smart decisions like adults can — but why?

Research into how the human brain develops helps explain. In a teenager, the frontal lobe of the brain, which controls decision-making, is built but not fully insulated — so signals move slowly.

"Teenagers are not as readily able to access their frontal lobe to say, 'Oh, I better not do this,' " Dr. Frances Jensen tells Fresh Air's Terry Gross.

Jensen, who's a neuroscientist and was a single mother of two boys who are now in their 20s, wrote The Teenage Brain to explore the science of how the brain grows — and why teenagers can be especially impulsive, moody and not very good at responsible decision-making.

"We have a natural insulation ... called myelin," she says. "It's a fat, and it takes time. Cells have to build myelin, and they grow it around the outside of these tracks, and that takes years."

This insulation process starts in the back of the brain and heads toward the front. Brains aren't fully mature until people are in their early 20s, possibly late 20s and maybe even beyond, Jensen says.

"The last place to be connected — to be fully myelinated — is the front of your brain," Jensen says. "And what's in the front? Your prefrontal cortex and your frontal cortex. These are areas where we have insight, empathy, these executive functions such as impulse control, risk-taking behavior."

This research also explains why teenagers can be especially susceptible to addictions — including drugs, alcohol, smoking and digital devices.